In many instances materials used as-coating, potting, and encapsulating materials e.g., gels must maintain adhesion to substrates. In electronics for example, gels are a special class of encapsulants that cure to form an extremely soft material. They are used to provide high levels of stress relief to sensitive circuitry. These materials may be used to perform many important functions in electronics. Their major function is to protect electronic assemblies and components from adverse environments by providing dielectric insulation, prevention of contaminant (e.g., moisture) ingress into/onto electronic circuitry and protection of electronic components from mechanical and thermal stress on components. In such situations the encapsulants are required to adhere to electronic and electrical components and printed circuit boards in addition to the electrical connectors and conductors that pass through the coating or encapsulating material.
Whilst a variety of organic based and silicone based encapsulants are available commercially, they each have disadvantages. For example:    (i) many commercially available silicone adhesives/encapsulants/pottants are not optically clear;    (ii) Epoxy based materials, polyurethane and silicones are known to discolour (e.g., yellow) with age;    (iii) Silicone based room temperature addition (hydrosilylation) cure materials do not provide sufficient adhesion to substrate surfaces;    (iv) In view of (iii) above, many silicone addition cure materials with adhesion promoters to enhance adhesion are required to cure at temperatures >100° C. (e.g., from 120-150° C.) which many electronic boards and the like cannot support; and    (v) Polyurethane or epoxy based materials are known to have high levels of shrinkage during cure which can create problems when used in large scale commercial situations.
The commercial silicone materials that are currently used to form the encapsulants and pottants etc. (e.g., gels) are expensive being based on addition cure chemistry, i.e., they are cured by the hydrosilylation/addition reaction of a silicon hydride group with an unsaturated carbon radical with the help of a catalyst, which is typically a platinum based compound. Historically the industry has preferred addition cure compositions of this type for these applications because they immediately cure throughout the body of the compound resulting in a cured material in a matter of minutes whilst condensation cure systems are significantly slower, titanate cured condensation processes can take e.g., up to 7 days curing per 6 mm of depth of the body of the uncured material. Tin cured condensation systems do cure over a shorter period but they are not desired for e.g., electronics applications because they undergo reversion (i.e., depolymerisation) at temperatures above 80° C.
Whilst from a cure speed standpoint materials made from hydrosilylation cure compositions are excellent there are several potential problems and/or disadvantages with their use. For example, they are generally cured at elevated temperature (i.e., in excess of 100° C.) and can be contaminated and rendered uncurable due to inactivation of expensive platinum based cure catalysts which are sensitive and may be poisoned by amine containing compounds, sulphur containing compounds and phosphorus containing compounds.
It is well known to people skilled in the art that alkoxy titanium compounds—i.e., alkyl titanates—are suitable catalysts for formulating one component moisture curable silicones (References: Noll, W.; Chemistry and Technology of Silicones, Academic Press Inc., New York, 1968, p. 399, Michael A. Brook, silicon in organic, organometallic and polymer chemistry, John Wiley & sons, Inc. (2000), p. 285). Titanate catalysts have been widely described for their use to formulate skin or diffusion cured one-part condensation curing silicone elastomers. These formulations are typically available in one-part packages that are applied in a layer that is thinner than typically 15 mm. Layers thicker than 15 mm are known to lead to uncured material in the depth of the material, because the moisture is very slow to diffuse in very deep sections. Skin or diffusion cure (e.g., moisture/condensation) takes place when the initial cure process takes place by the formation of a cured skin at the composition/air interface subsequent to the sealant/encapsulant being applied on to a substrate surface. Subsequent to the generation of the surface skin the cure speed is dependent on the speed of diffusion of moisture from the sealant/encapsulant interface with air to the inside (or core), and the diffusion of condensation reaction by-product/effluent from the inside (or core) to the outside (or surface) of the material and the gradual thickening of the cured skin over time from the outside/surface to the inside/core.
Multi component compositions designed to activate condensation cure in the bulk of the product do not use titanium based catalysts. They generally use other metal catalysts such as tin or zinc catalyst, e.g., dibutyl tin dilaurate, tin octoate and/or zinc octoate (Noll, W.; Chemistry and Technology of Silicones, Academic Press Inc., New York, 1968, p. 397). In silicone compositions stored before use in two or more parts, one-part contains a filler which typically contains the moisture required to activate condensation cure in the bulk of the product. Unlike the previously mentioned diffusion cure one-part system, two-part condensation cure systems, once mixed together, enable bulk cure even in sections greater than 15 mm in depth. In this case the composition will cure (subsequent to mixing) throughout the material bulk. If a skin is formed, it will be only in the first minutes after application. Soon after, the product will become a solid in the entire mass. Titanate catalysts are not used for curing these types of two part compositions because it is well known that in the presence of a significant amount of moisture alkyl titanate catalysts will fully hydrolyse to form tetrahydroxy titanate, which is insoluble in silicone. This form of titanium loses its catalytic efficiency, leading to uncured systems. Hence, there is a need for an optically clear, room temperature curable, LED lighting encapsulation/potting material.